1. Field of the Invention
The present invention relates to a camera and, more particularly, it relates to a camera of lens-interchangeable type including a camera body and an interchangeable lens in which an electric connection of the interchangeable lens with the camera body is improved.
2. Description of the Related Art
Recently an automatic camera has become widely used. Such an automatic camera includes an automatic exposure control device, an automatic focusing device and/or an automatic light adjusting device, so that various electronic circuits, motor, battery or the like are mounted in the camera.
In the case of a camera of lens-interchangeable type, including a camera body and an interchangeable lens, in which various devices as mentioned above are contained, a motor, electronic circuits and the like are arranged in the camera body as well as in the interchangeable lens. Accordingly, electric contacts or connecting terminals for electrically connecting the various device in the camera body with those in the interchangeable lens are arranged on mount portions for connecting the camera body with the lens.
In the conventional camera of lens-interchangeable type, it has been the usual practice to arrange a ground line and a connecting terminal for the motor mounted in the lens and those for the electronic circuit so that they are commonly used, in order to reduce the number of connecting terminals. However, in the system in which the ground lines and the connecting terminals are arranged for common use, there is a danger that the electronic circuits of the lens and the camera body may be damaged owing to difference in ground potential between them.
For example, in the construction wherein electric power is transmitted from a power source in the camera through a power supply line and a terminal to the lens to supply the power to a motor circuit and/or an electronic circuit in the lens, at the time of supplying the power to the motor circuit the power is fed from the power source in the camera body through said power line to the motor circuit and then returned through the ground line and the terminal to said power source, so that a closed loop is formed to provide a power feeding path for the motor circuit. In this case, the current passing through the motor circuit is relatively large, so that said current produces a voltage drop at the ground line and the terminal when it is returned through said ground line and said terminal to the power source. If this ground line was commonly used in the motor circuit and the electronic circuit of the lens, the ground potential of the electronic circuit would be increased, by the amount of the above-mentioned voltage drop, as compared with the normal level, with the result that the electronic circuit of the lens would be adversely affected thereby.
Furthermore, in a camera of the above-mentioned type, if the lens was not properly mounted on the camera body, the contact terminals of the camera body and the lens would not be properly connected. Such a state would cause an accident wherein the lens could not be driven even if an instruction was sent from the camera body to the lens to effect some operation, such as driving of the motor and, moreover, such a state could produce various electrical accidents.
In order to avoid such accidents, a device for detecting the mounting of the lens, which is constructed to allow transmitting and receiving signals between the camera body and the lens only when the lens is properly mounted on the camera body, has been proposed, for example, in U.S. Pat. No. 4,492,429 and Pat. No. 4,464,034, etc. The device as proposed in these patents includes a switch associated with a mount locking member, which is so constructed that when the lens has been completely mounted on the camera body the mount locking member is pressed by the mount of the lens, with the result that said switch is actuated by said mount locking member.
However, according to the proposed construction in which the operation of the switch is effected by the mount locking member, it is quite possible that an operator may erroneously unlock said mount locking member owing to his careless operation. If the operator erroneously unlocked the mount locking member, the feeding of power to the electronic circuit in the lens would be broken, so that the transmitting and receiving signals between the camera body and the lens would become impossible, thereby making the operation of the camera itself impossible.